Update the new permission layer to be alterable

Nice work!
Functionally tested multiple scenarios all pass very nicely.
- Added new group types
- Changed permissions for roles
- Changed roles on group members

All had the desired effect on the access of the group and it's entities.

A couple of tiny nitpicks but other than that works like a charm.

1. +++ b/src/Access/DefaultGroupPermissionCalculator.php
   @@ -0,0 +1,144 @@
   +    foreach ($this->membershipLoader->loadByUser($account) as $group_membership) {
   +      $group_id = $group_membership->getGroup()->id();
   +      $permissions = [];
   +
   +      foreach ($group_membership->getRoles() as $group_role) {
   +        $permissions = array_merge($permissions, $group_role->getPermissions());
   +        $calculated_permissions->addCacheableDependency($group_role);
   +      }
   

   We'd be better of not using array_merge in this loop. Using that in a loop could be very costly.

2. +++ b/src/Access/SynchronizedGroupPermissionCalculator.php
   @@ -0,0 +1,124 @@
   +      if (!empty($group_role_ids)) {
   +        /** @var \Drupal\group\Entity\GroupRoleInterface $group_role */
   +        foreach ($group_role_storage->loadMultiple($group_role_ids) as $group_role) {
   +          $permissions = array_merge($permissions, $group_role->getPermissions());
   +          $calculated_permissions->addCacheableDependency($group_role);
   +        }
   +      }
   

   Similar as the above

3. +++ b/src/Access/SynchronizedGroupPermissionCalculator.php
   @@ -0,0 +1,124 @@
   +    foreach ($group_type_storage->loadMultiple() as $group_type_id => $group_type) {
   

   $group_type seems to be an unused variable.

Will leave it at needs review for now, can imagine others are keen to test this out as well.

D8.8 requires php7? Maybe need to fix the projects drupal-ci yml